In present pulp mills, as well as those contemplated in the future, there are and will be liquid flows which are contaminated by various metal ions, which can disturb the bleaching reactions which take place during bleaching with hydrogen peroxide or ozone. These liquid flows occur, for example, during totally chlorine-free bleaching, so-called TCF-bleaching (TCF=Totally Chlorine Free), when the pulp is treated with chelating agents, such as EDTA or DTPA, in order to substantially reduce the metal ion content of the pulp prior to its bleaching with peroxide or ozone. This type of bleaching, in order to yield high ISO-brightness, requires that both the pulp and the process water be free or substantially free of certain metal ions since otherwise the charged hydrogen peroxide will effectively disintegrate to water and oxygen gas, and the charged ozone to oxygen gas, without having any simultaneous bleaching effect.
Particularly at those times when the pulp mill is closed, for example for environmental reasons, these liquids containing metal ions can be very difficult to deal with in order to prevent their contact with the bleaching step which would then deteriorate bleaching efficiency.